The invention relates to a planar optical waveguide having a crystal body consisting of a nonmagnetic substrate having a first magnetooptical layer epitaxially provided on the substrate and a second magnetooptical layer epitaxially provided on the first magnetooptical layer. The two magnetooptical layers consist of a material based on iron garnet. The second magnetooptical layer has a refractive index n.sub.2 which is approximately 1.times.10.sup.-3 to 1.times.10.sup.-2 higher than the refractive index n.sub.1 of the first magnetooptical layer.
The invention further relates to a method of manufacturing an optical waveguide of this type and to its use.
In optical communication by means of optical glass fibres, one-way waveguides are used as optical isolators or possibly also as optical circulators to protect the semiconductor laser diodes from light reflected from the connected section. The nonreciprocal properties of magnetooptical materials are utilized on account of the Faraday effect.
Planar optical one-way waveguides which are realized by layers of a magnetooptical material provided epitaxially on a substrate must have a layer thickness of the radiation-conducting layer which is adapted to the dimensions of the optical fibres to be connected. Monomode optical fibres have, for example, a core diameter in the range of 5 to 10 .mu.m. Therefore, the layer thickness of the radiation-conducting layer of a monomode waveguide should also be in the range of 5 to 10 .mu.m.
Layers of these dimensions can be manufactured by means of liquid phase epitaxy (LPE) from highly diluted molten solutions in which the solvent conventionally consists of a mixture of PbO and B.sub.2 O.sub.3. Suitable as a material for the magnetooptical layer is, for example, yttrium iron garnet (Y.sub.3 Fe.sub.5 O.sub.12) and suitable for the substrate on which such layers grow epitaxially is, for example, gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12) in the form of a commercially available 0.5 mm thick single crystal slice oriented in the (111)-direction. For the magnetooptical layers other iron garnets are also suitable such as, for example, Gd.sub.3 Fe.sub.5 O.sub.12 or bismuth-substituted iron garnets.
In optical communication by means of optical fibres, the monomode technique is used for a high-grade data transmission. The planar optical waveguide referred to in the opening paragraph operates with two magnetooptical iron garnet layers having a small refractive index difference between the two magneto-optical layers, so that the second iron garnet layer adjoining the first layer provided on the substrate is a monomode waveguide in the case of a thickness of 5 to 10 .mu.m.
Due to the relatively large refractive index difference between the substrate and the first magnetooptical layer further modes can be additionally propagated in the entire magnetooptical double layer. For an optimum monomode operation this effect is unwanted.